Buried via in a circuit board

ABSTRACT

A method may include forming a plurality of multilayer cores wherein each multilayer core comprises a sheet of cured dielectric material having a layer of metal on each side of the sheet of cured dielectric material, patterning each layer of metal in the plurality of multilayer cores to form wiring traces in each layer of metal, embedding a solder element in at least one sheet of a plurality of sheets of uncured dielectric material, wherein the solder element having a melting point temperature within a temperature range of a curing temperature of the uncured dielectric material, forming a printed circuit board by alternately stacking the plurality of multilayer cores with the plurality of sheets of uncured dielectric material between each multilayer core, laminating the stack of multilayer cores and sheets of uncured dielectric material to cause curing of the sheets of uncured dielectric material and melting of the solder element.

BACKGROUND

The present invention relates generally to a printed circuit board, andmore particularly, to a structure and a method of forming a printedcircuit board having buried vias.

A multilayer printed circuit board may consist of many layers and vias.The layers may include dielectric layers and metal layers. The vias mayprovide an electrical current between layers in the printed circuitboard. The printed circuit board may have both back and frontmetallization, typically referred to as circuit trances. In addition,the printed circuit board may include ground planes and othermetallization structures.

To create a via, holes may be drilled into the printed circuit board andthe internal walls may be electroplated or otherwise metallized. Basictypes of vias for a printed circuit board may include through vias,blind vias, and buried vias. Through vias may extend fully between outersurfaces of the printed circuit board. The blind vias may penetrate fromone outer surface of the printed circuit board to a specified depth inthe printed circuit board stack and stop. The buried vias may exist onlyin the intermediate layers of the printed circuit board stack and arenot accessible from the outer surfaces of the printed circuit board.

SUMMARY

According to one embodiment of the present invention, a method isprovided. The method may include forming a plurality of multilayer coreswherein each multilayer core comprises a sheet of cured dielectricmaterial having a layer of metal on each side of the sheet of cureddielectric material, patterning each layer of metal in the plurality ofmultilayer cores to form wiring traces in each layer of metal, embeddinga solder element in at least one sheet of a plurality of sheets ofuncured dielectric material, wherein the solder element having a meltingpoint temperature within a temperature range of a curing temperature ofthe uncured dielectric material, forming a printed circuit board byalternately stacking the plurality of multilayer cores with theplurality of sheets of uncured dielectric material between eachmultilayer core, where at least one sheet of the uncured dielectricmaterial having the embedded solder element positioned so as to bealigned with the wiring traces in adjacent layers of metal in adjacentmultilayer cores, and laminating the stack of multilayer cores andsheets of uncured dielectric material to cause curing of the sheets ofuncured dielectric material and melting of the solder element.

According to another embodiment of the present invention, a method isprovided. The method may include forming a plurality of multilayer coreswherein each multilayer core comprises a sheet of cured dielectricmaterial having a layer of metal on each side of the sheet of cureddielectric material, patterning each layer of metal in the plurality ofmultilayer cores to form wiring traces in each layer of metal, embeddinga solder element in at least one sheet of a plurality of sheets ofuncured dielectric material and at least one multilayer core, whereinthe solder element having a melting point temperature within atemperature range of a curing temperature of the uncured dielectricmaterial, forming a printed circuit board by alternately stacking one ofthe plurality of multilayer cores with one of the plurality of sheets ofuncured dielectric material between each multilayer core, the at leastone sheet of the uncured dielectric material having the embedded solderelement positioned so as to be aligned with the wiring traces inadjacent layers of metal in adjacent multilayer cores and the at leastone multilayer core having the embedded solder element positioned so asto be aligned with the solder element in the adjacent uncured dielectricmaterial, and laminating the stack of multilayer cores and sheets ofuncured dielectric material to cause curing of the sheets of uncureddielectric material and melting of the solder element.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and notintend to limit the invention solely thereto, will best be appreciatedin conjunction with the accompanying drawings, in which:

FIG. 1 is a cross section view illustrating a multilayer core of aprinted circuit board according to an exemplary embodiment;

FIG. 2 is a cross section view illustrating a prepreg layer of a printedcircuit board according to an exemplary embodiment;

FIG. 3 is a cross section view illustrating a solder element of aprinted circuit board according to an exemplary embodiment;

FIG. 4 is a cross section view illustrating several examples of anetched multilayer core according to an exemplary embodiment;

FIG. 5 is a cross section view illustrating two types of prepreg layersaccording to an exemplary embodiment;

FIG. 6 is a cross section view illustrating cores stacked with prepreglayers according to an exemplary embodiment;

FIG. 7 is a cross section view illustrating the printed circuit boardwith cured prepreg layers according to an exemplary embodiment;

FIG. 8 is a cross section view illustrating forming a through via in theprinted circuit board according to an exemplary embodiment;

FIG. 9 is a cross section view illustrating metallization of the throughvia according to an exemplary embodiment; and

FIG. 10 is a cross section view illustrating an alternative embodimentof a printed circuit board according to an exemplary embodiment.

The drawings are not necessarily to scale. The drawings are merelyschematic representations, not intended to portray specific parametersof the invention. The drawings are intended to depict only typicalembodiments of the invention. In the drawings, like numbering representslike elements.

DETAILED DESCRIPTION

Detailed embodiments of the claimed structures and methods are disclosedherein; however, it can be understood that the disclosed embodiments aremerely illustrative of the claimed structures and methods that may beembodied in various forms. This invention may, however, be embodied inmany different forms and should not be construed as limited to theexemplary embodiment set forth herein. Rather, these exemplaryembodiments are provided so that this disclosure will be thorough andcomplete and will fully convey the scope of this invention to thoseskilled in the art. In the description, details of well-known featuresand techniques may be omitted to avoid unnecessarily obscuring thepresented embodiments.

For purposes of the description hereinafter, the terms “upper”, “lower”,“right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, andderivatives thereof shall relate to the disclosed structures andmethods, as oriented in the drawing figures. The terms “overlying”,“atop”, “on top”, “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements, such as aninterface structure may be present between the first element and thesecond element. The term “direct contact” means that a first element,such as a first structure, and a second element, such as a secondstructure, are connected without any intermediary conducting, insulatingor semiconductor layers at the interface of the two elements.

In the interest of not obscuring the presentation of embodiments of thepresent invention, in the following detailed description, someprocessing steps or operations that are known in the art may have beencombined together for presentation and for illustration purposes and insome instances may have not been described in detail. In otherinstances, some processing steps or operations that are known in the artmay not be described at all. It should be understood that the followingdescription is rather focused on the distinctive features or elements ofvarious embodiments of the present invention.

Embodiments of the present invention generally relate to a printedcircuit board, and more particularly, to a structure and a method offorming a printed circuit board having buried vias. Generally, a printedcircuit board is made up of layers of prepreg and core. A prepreg may bea sheet of uncured dielectric material. A core may be a sheet of cureddielectric material with layers of a metal, such as, for example,copper, on either side. The printed circuit board is made up of stacksof the core with the prepreg sheet separating each core.

The printed circuit board may have many vias, such as, for example,through vias, blind vias, and buried vias. To create a via, such as athrough via, a hole may be drilled through the stacks of the core andprepreg layers of the printed circuit board. The hole extends throughoutall the stacks of the core and the prepreg layers. The hole may beelectroplated or otherwise metalized, creating the through via.

Creating buried vias is more time consuming, expensive, and laborintensive because using conventional processes, most buried vias arefirst constructed as through vias. For example, the printed circuitboard may be made up of three stacks of the core. One stack of the coremay be used to create a through via. Once the through via is created,all three stacks of the core are assembled such that the stack with thethrough via is placed in the middle with each of the remaining twostacks of the core being stacked on top and below the stack with thethrough via. In addition, in between each stack of the core, the prepreglayer is deposited. The resultant structure is a printed circuit boardwith a buried via. The buried via is the through via of the one of thethree stacks, buried in between the other two stacks of the core.

Buried vias may also be constructed using a solder element inconjunction with the prepreg layer. In that particular method, thesolder element is inserted into the prepreg layer before the printedcircuit board is constructed. The prepreg layers with the solderelements, along with the stack of cores, are then used to construct theprinted circuit board, where each stack of cores is separated by theprepreg layer with the solder element. The resultant structure undergoesa heating process followed by a lamination process. During the heatingprocess, a heat sources, such as, for example, a laser, is used tolocally heat the solder elements, melting them. Once the solder elementmelts and subsequently solidifies, the solder element is the buried via.

After the solder elements are melted, the printed circuit boardundergoes a lamination process to laminate the core stacks together withthe prepreg. Using this method is requires multiple steps, ischallenging, and expensive because the laser must be designed to be usedin printed circuit board manufacturing. Accordingly, there exists a needto reduce the manufacturing cost of printed circuit boards with buriedvias.

Embodiments of the present invention provide a method of creating buriedvias in a printed circuit board that is less expensive, not as timeconsuming and labor intensive as the conventional process. Embodimentsof the present invention provide a method that reduces the number ofheating steps needed and does not require special equipment, such as,for example, a laser to heat the solder element, creating the buriedvia. In addition, the new method does not require the buried via to passthrough a core when compared to the conventional method where the buriedvia must pass through a core. Further, embodiments of the presentinvention provide a design improvement by allowing blind vias to overlapin a multilayer printed circuit board.

Referring now to FIG. 1, a multilayer core 10 is shown, in accordancewith an embodiment. The multilayer core 10 may be formed in sheetsapproximating the size of the finished printed circuit board.Alternatively, the multilayer core 10 may be made larger than thefinished printed circuit board and then cut to size. The multilayer core10 may include a cured dielectric layer 12, a first metal layer 14, anda second metal layer 16.

The cured dielectric layer 12 may be made of any material having arelative permativity (ER) of about 3.2 to 4.8. The cured dielectriclayer 12 may be made of uncured dielectric material that has been maderigid through a curing process, thus forming the cured dielectric layer12. The uncured dielectric material that may be used to form the cureddielectric layer 12 may be made up of woven glass cloth impregnated withand surrounded by resin. One example of a suitable dielectric may be aresin-glass weave, such as, for example, FR4 which may include a wovenfiberglass cloth with an epoxy resin binder. Another example of asuitable dielectric may be a resin such as ceramic filledpolytetrafluoroethylene (PTFE) composite. The curing of the dielectricmaterial causes the resin to become rigid.

The first metal layer 14 is deposited on the top surface of the cureddielectric layer 12. The second metal layer 16 is deposited on thebottom surface of the cured dielectric layer 12. As a result, the topsurface of the cured dielectric layer 12 is in direct contact with thefirst metal layer 14 and the bottom surface of the cured dielectriclayer 12 is in direct contact with the second metal layer 16. Knownsuitable deposition techniques, such as, for example, atomic layerdeposition, chemical vapor deposition, or physical vapor deposition maybe used to form the core 10.

The first metal layer 14 and the second metal layer 16 may be made ofmetal, such as, for example, copper, gold, nickel, gold-nickel alloy, orany other suitable metal. The resultant core 10 includes the cureddielectric layer 12 sandwiched between the first metal layer 14 and thesecond metal layer 16.

Referring now to FIG. 2, a prepreg layer 18 is shown, in accordance withan embodiment. The prepreg layer 18 may also be referred to as anuncured dielectric layer because the prepreg layer 18 may be made of thesame materials that make up the cured dielectric layer 12. Some examplesof material that make up the prepreg layer 18 may include, but are notlimited to, FR4 which may include a woven fiberglass cloth with an epoxyresin binder, or a ceramic filled PTFE composite. In addition, since theprepreg layer 18 is made of uncured dielectric material, the resincontained in the uncured dielectric material may have the consistency ofa highly viscous paste with some stability provided by the glass weave.

Referring now to FIG. 3, a solder element 20 is shown, in accordancewith an embodiment. The solder element 20 may be a homogeneous solderalloy preform that may come in standard shapes such as, for example,squares or rectangles, and which may contain a flux. The composition ofthe solder element 20 may be selected from a group of alloys. The alloysmay have a melting temperature to be within a few degrees of the curingtemperature of the prepreg layer 18. For example, the alloy that makesup the solder element 20 may have its melting point within 5° C. of thecuring temperature of the prepreg layer 18.

The solder element 20 may also be a solder paste preform. The solderpaste preform may contain various elements of the solder alloy and abinder, which may also be the flux. The solder paste may not bepreviously melted. The solder composition may have a liquidustemperature to be within 5° C. of the curing temperature of the prepreglayer 18. The solder element 20 may be embedded in the prepreg layer 18during the formation of the printed circuit board.

Embodiments of the invention contemplate using the prepreg layers 18A inconjunction with the solder elements 20. Further, embodiments of theinvention contemplate having the solder element 20 made of materialwhose melting temperature is similar to the temperature range of thecuring temperature of the prepreg layer 18. In addition, it is criticalthat the melting temperature of the solder element 20 is lower than thecuring temperature of the prepreg layer 18. Designing the meltingtemperature of the solder element 20 to be similar to the curingtemperature of the prepreg layer 18, while also being lower than thecuring temperature of the prepreg layer 18, eliminates the step ofheating the solder element 20 using a laser. As a result, a singlelamination step can be used to melt the solder element 20 and cure theprepreg layer 18, as will be described in more detail hereafter withrespect to FIG. 7.

In an embodiment, the melting temperature of the solder element 20 issimilar to the curing temperature of the prepreg layer 18 when thedifference between the melting temperature of the solder element 20 andthe curing temperature of the prepreg layer 18 ranges from approximately0° C. to approximately 20° C. For example, in an embodiment, the solderelement 20 may be made of material that has a melting point temperatureof 185° C. and the prepreg layer 18 may be made of material that has acuring temperature of 205° C.

In another embodiment, the difference between the melting temperature ofthe solder element 20 and the curing temperature of the prepreg layer 18may range from approximately 0° C. to approximately 5° C. For example,in an embodiment, the solder element 20 may be made of material that hasa melting point temperature of 190° C. and the prepreg layer 18 may bemade of material that has a curing temperature of 195° C. The differencebetween the two temperatures is 5° C. In addition, the melting point ofthe solder element 20 is lower than the curing temperature of theprepreg layer 18. Having the difference between the melting temperatureof the solder element 20 and the curing temperature of the prepreg layer18 range from approximately 0° C. to approximately 5° C. may shorten thelamination process time. In addition, it may produce buried vias thathave better connectivity when compared to the buried vias that areproduced when the difference between the two temperatures ranges fromapproximately 10° C. to approximately 20° C.

Referring now to FIG. 4, four different multilayer cores 10A, 10B, 10C,and 10D are shown, in accordance with an embodiment. The first metallayer 14 and the second metal layer 16 of the multilayer core 10,illustrated in FIG. 1, may be patterned, using known etching techniques,such as, for example, wet etch, to remove portions of the metal layers14 and 16. When portions of the metal layers 14 and 16 are etched,wiring traces 22 and 24 are formed. The formation of the wiring traces22, 24 may vary, depending on the wiring requirements of the printedcircuit board.

In order to achieve the desired wiring traces 22, 24, portions of themetal layers 14 and 16 are masked and the remaining unmasked portions ofthe metal layers 14 and 16 are etched to expose the top and bottomsurfaces of the cured dielectric layer 12. It should be understood thateven though four different multilayer cores are shown in FIG. 4 in whichthe wiring on the cured dielectric layer 12 varies, embodiments of theinvention are not limited to the four. Other variations of the wiringtraces 22, 24 may be contemplated.

Referring now to FIG. 5, two types of prepreg layers 18A and 18B areshown, in accordance with an embodiment. Prepreg layer 18A may have aperforation 26 in it whereas the prepreg layer 18B has no suchperforation 26. The perforation 26 may be formed, for example, bydrilling the prepreg layer 18B, thus forming prepreg layer 18A. Theperforation 26 may extend entirely through the prepreg layer 18A. Duringthe construction of the printed circuit board, the solder element 20(illustrated in FIG. 3) may be deposited in the perforation 26. Asdescribed herein above, the prepreg layers 18A, 18B may be made ofuncured dielectric material.

Referring now to FIG. 6, a printed circuit board 30 is shown, inaccordance with an embodiment. The printed circuit board 30 may includea plurality of cores 10 and prepreg layers 18. The cores 10 are threeexamples of the cores illustrated in FIG. 4 and include core 10A, core10C and core 10D. The cores 10 may be stacked together, one core 10 ontop of another core 10 with the prepreg layers 18A interspersed betweenthe cores 10.

In an embodiment, between the multilayer core 10D and the multilayercore 10A, the prepreg layer 18A may be deposited, using known depositiontechniques. The perforation 26 of the prepreg layer 18A may be filled bythe solder element 20. Similarly, between the multilayer core 10A andthe multilayer core 10C, another prepreg layer 18A may be deposited andthe perforation 26 of that prepreg layer 18A may again be filled withthe solder element 20. The solder element 20 may be positioned in theprepreg layer 18A such that the solder element 20 may be aligned withthe wiring traces 22 and 24 in the adjacent layers of the multilayercores 10.

In an embodiment, the solder elements 20 may entirely fill theperforations 26 in the prepreg layers 18A. In an alternative embodiment,there may be a small gap between the solder element 20 and the prepreglayer 18A.

The prepreg layers 18B may be placed on top of the stack and on thebottom of the stack. As a result, the prepreg layer 18B may be placed ontop of the core 10D and another prepreg layer 18B may be placed on thebottom of the core 10C. In an alternative embodiment, the printedcircuit board 30 may have both prepreg layer 18A and prepreg layer 18Binterspersed between the cores 10.

In an embodiment, the circuit board 30, illustrated in FIGS. 6 to 10,does not call for a core such as core 10B (illustrated in FIG. 4).However, in an alternative embodiment, the printed circuit board 30 mayhave the core 10B in its stack.

It should be understood that three multilayer cores 10 have been shownin FIG. 6 and subsequent FIGs. for the purpose of illustration and notlimitation. The stack for a printed circuit board may contain more orless than the three cores 10. Also, FIG. 6 and subsequent FIGs. showonly two solder elements 20 for the purpose of illustration and notlimitation. In practice, there may be many more than two solder elements20 within the printed circuit board 30 including multiple solderelements 20 within each prepreg layer 18A in the printed circuit board30. Finally, FIG. 6 and subsequent FIGs. show the solder elements 20being vertically aligned for the purpose of illustration and notlimitation. As there is expected in practice to be many more solderelements, it is expected that there may not be vertical alignmentbetween the solder elements 20. In various layers the solder elements 20may be spaced horizontally with respect to the solder element above orbelow and may even overlap.

Referring now to FIG. 7, the printed circuit board 30 is shown after theprinted circuit board 30 has undergone a lamination process, inaccordance with an embodiment. During the lamination process, heat andpressure are applied to the printed circuit board 30. The printedcircuit board 30 gets heated to a temperature that allows for theprepreg layers 18A and 18B to cure and the solder element 20 to melt.During the lamination process, the prepreg layers 18A and 18B are pushedinto the gaps between the wiring trances in the metal layers 14 and 16,effectively filling the wiring traces. In addition, the laminationprocess also heats the solder elements 20 and melts the solder elements20. The melting of the solder elements 20 creates electrical continuitybetween the metal traces 22, and 24. Once the lamination processfinishes, the solder elements 20 may solidify.

Generally, when using a conventional method of forming a buried viamultiple heating steps are required. The lamination process is firstpreceded by a heating step during which only the solder elements arelocally heated by a laser. The heating of the solder elements causesthem to melt. Once the laser is removed, the solder elements solidifyforming the buried vias. After the formation of the buried vias, theprinted circuit board undergoes a lamination process. During thelamination process, the printed circuit board gets heated and pressureis applied to cure the prepreg layers. The lamination process may havean adverse effect on the solidified solder elements causing the solderelements to crack. As a result, the lamination process may be followedby another heating step, during which a laser is used to re-melt thesolder elements that may have cracked during the lamination process.

When using the conventional method to form the buried via, thelamination step is followed by localized heating of the solder elementusing the laser. Placing the laminated printed circuit board in an ovento re-melt the solder element is not beneficial. Since the laminatedprinted circuit board has layers of cured dielectric and cured prepreg,the printed circuit board cannot be heated to a temperature high enoughthat may re-melt the solder element because the printed circuit boardmay burn. As a result, re-melting the solder element after lamination isdone using a laser.

A benefit of the embodiments of the present invention is that theprinted circuit board 30 undergoes only one heating process, thelamination process, to form the buried via. Since the solder elements 20melt at a temperature that is close in temperature range to the curingtemperature of the prepreg layers 18A and 18B, the printed circuit board30 undergoes only one heating process. During the lamination process theprinted circuit board 30 gets heated and pressure is applied, causingthe prepreg layers 18A and 18B to cure and the solder elements 20 tomelt. Once the lamination process ends, the solder elements 20 solidify,creating buried vias.

Further, heating the entire printed circuit board 30 during the singlelamination process does not necessitate high temperatures because thesolder elements 20 and the prepreg layers 18A and 18B may be made ofmaterials whose melting point and curing temperatures, respectively,below 300° C. For example, the melting point of the solder elements 20and the curing temperatures of the prepreg layers 18A and 18B may be inthe range of 180° C.-230° C.

Embodiments of the present invention contemplate matching the meltingpoint temperatures of the solder elements 20 to the curing temperaturesof the prepreg layers 18A and 18B. The solder element 20 may not meltfaster or slower than the curing of the prepreg layer 18 because thatmay have an adverse effect on the buried via and the connection it maymake with the surrounding layers. As a result, the solder element 20 ismade of material that allows for the solder element 20 to meltconcurrently with the curing of the prepreg layer 18.

Referring now to FIG. 8, the printed circuit board 30 that has a throughvia opening 34 is shown, in accordance with an embodiment. After thelamination process, the through via opening 34 may be formed, forexample, by drilling a hole in the printed circuit board 30. The throughvia opening 34 extends from the top of the printed circuit board 30 tothe bottom of the printed circuit board 30. The through via opening 34may be formed to provide an electrical current between elementsconnected to the printed circuit board 30.

Referring now to FIG. 9, the printed circuit board 30 with a through via36 is shown, in accordance with an embodiment. Once the through viaopening 34 is formed in the printed circuit board 30, the through viaopening 34 may be subsequently plated, with a metal, such as, forexample, copper, creating the through via 36. The top portion and thebottom portion of the exposed prepreg layers 18B may also be plated withthe same metal as the through via opening 34. Known plating techniques,such as, for example, electroplating, may be used to plate the throughvia opening 34 and the top and bottom portions of the exposed prepreglayers 18B.

After the top and bottom portions of the exposed prepreg layers 18B areplated, they may be chemically etched to form microstrip features 38.The microstrip features 38 may be subsequently used current paths whenother features are connected to the printed circuit board 30.

Referring now to FIG. 10, another exemplary embodiment of a printedcircuit board 70 is shown, in accordance with an embodiment. In theprior exemplary embodiment of printed circuit board 30, the perforations26 and solder elements 20 were only in the prepreg layers 18A and didnot extend through any of the cores 10. In the exemplary embodiment ofthe printed circuit board 70 the perforation 66 and the solder core 68extend through one or more cores 10 and one or more prepreg layers 18A.

The printed circuit board 70 may be formed in substantially the same wayas the printed circuit board 30. The etched cores 10 and the prepreglayers 18A are stacked on top of one another. During the stacking, ahole may be drilled through one of the cores 10 and the surroundingprepreg layers 18A. The hole may be then filled with the solder core 68.Additional cores 10 and prepreg layers 18A or 18B may be deposited ontop. The printed circuit board 70 undergoes a lamination process duringwhich the prepreg layers 18A and 18B are cured and the solder core 68 ismelted.

As illustrated in FIG. 10, the perforation 66 and the solder core 68 mayextend through the core 10A and the surrounding prepreg layers 18A.Having the solder core 68 extend through one of the cores 10 translatesto a buried via that may span across multiple inter layers. This allowsfor multiple layer connectivity.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiment, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method comprising: forming a plurality ofmultilayer cores wherein each multilayer core comprises a sheet of cureddielectric material having a layer of metal on each side of the sheet ofcured dielectric material; patterning each layer of metal in theplurality of multilayer cores to form wiring traces in each layer ofmetal; embedding a solder element in at least one sheet of a pluralityof sheets of uncured dielectric material, wherein the solder elementhaving a melting point temperature within a temperature range of acuring temperature of the uncured dielectric material; forming a printedcircuit board by alternately stacking the plurality of multilayer coreswith the plurality of sheets of uncured dielectric material between eachmultilayer core, the at least one sheet of the uncured dielectricmaterial having the embedded solder element positioned so as to bealigned with the wiring traces in adjacent layers of metal in adjacentmultilayer cores; and laminating the stack of multilayer cores andsheets of uncured dielectric material to cause curing of the sheets ofuncured dielectric material and melting of the solder element.
 2. Themethod of claim 1, further comprising: forming a through via openingthrough the stack of multilayer cores and sheets of cured dielectricmaterial; and metallizing the through via opening.
 3. The method ofclaim 1, wherein the metal is copper.
 4. The method of claim 1, whereinthe solder element has the melting point temperature within 5° C. of thecuring temperature of the uncured dielectric material.
 5. The method ofclaim 1, wherein the solder element has the melting point temperaturethat is below the curing temperature of the uncured dielectric material.6. The method of claim 1, wherein the solder element comprises solderand a flux.
 7. The method of claim 1, wherein embedding the solderelement in the at least one sheet of the uncured dielectric material isonly in the at least one sheet of the uncured dielectric material andnot in the adjacent multilayer cores.
 8. A method comprising: forming aplurality of multilayer cores wherein each multilayer core comprises asheet of cured dielectric material having a layer of metal on each sideof the sheet of cured dielectric material; patterning each layer ofmetal in the plurality of multilayer cores to form wiring traces in eachlayer of metal; embedding a solder element in at least one sheet of aplurality of sheets of uncured dielectric material and at least onemultilayer core, wherein the solder element having a melting pointtemperature within a temperature range of a curing temperature of theuncured dielectric material; forming a printed circuit board byalternately stacking one of the plurality of multilayer cores with oneof the plurality of sheets of uncured dielectric material between eachmultilayer core, the at least one sheet of the uncured dielectricmaterial having the embedded solder element positioned so as to bealigned with the wiring traces in adjacent layers of metal in adjacentmultilayer cores and the at least one multilayer core having theembedded solder element positioned so as to be aligned with the solderelement in the adjacent uncured dielectric material; and laminating thestack of multilayer cores and sheets of uncured dielectric material tocause curing of the sheets of uncured dielectric material and melting ofthe solder element.
 9. The method of claim 8, further comprising:forming a through via opening through the stack of multilayer cores andsheets of cured dielectric material; and metallizing the through viaopening.
 10. The method of claim 8, wherein the solder element has themelting point temperature within 5° C. of the curing temperature of theuncured dielectric material.
 11. The method of claim 8, wherein thesolder element has the melting point temperature that is below thecuring temperature of the uncured dielectric material.
 12. A methodcomprising: forming a plurality of multilayer cores wherein eachmultilayer core comprises a sheet of cured dielectric material having alayer of metal on each side of the sheet of cured dielectric material;patterning each layer of metal in the plurality of multilayer cores toform wiring traces in each layer of metal; forming a plurality of sheetsof uncured dielectric material; embedding a solder element in at leastone sheet of a plurality of sheets of uncured dielectric material,wherein the solder element having a melting point temperature within atemperature range of a curing temperature of the uncured dielectricmaterial; forming a printed circuit board by alternately stacking theplurality of multilayer cores with the plurality of sheets of uncureddielectric material between each multilayer core, the at least one sheetof the uncured dielectric material having the embedded solder elementpositioned so as to be aligned with the wiring traces in adjacent layersof metal in adjacent multilayer cores; and laminating the stack ofmultilayer cores and sheets of uncured dielectric material to causecuring of the sheets of uncured dielectric material and melting of thesolder element.
 13. The method of claim 12, further comprising: forminga through via opening through the stack of multilayer cores and sheetsof cured dielectric material; and metallizing the through via opening.14. The method of claim 12, wherein the solder element has the meltingpoint temperature within 5° C. of the curing temperature of the uncureddielectric material.
 15. The method of claim 12, wherein the solderelement has the melting point temperature that is below the curingtemperature of the uncured dielectric material.